Devices which can detect the presence of particles in fluids, including air, are well known, and are widely used in manufacturing industries. The most sensitive of these devices employ a light scattering method. As mobile particles pass through a light beam, light is scattered by the particles and received by a detector. Electronics associated with the detector then process the scattered light signal to determine factors such as the size or number of particles present.
The detection of mobile particles is especially important to semiconductor manufacturing processes which often takes place in strictly controlled environments such as "clean rooms". In such environments, mobile particles can pose a serious threat to the integrity of manufacturing processes. For example, unwanted particulate matter deposited on substrates can open or short circuits, interfere with deposition of metal films, or result in poisoned active regions. Furthermore, the fabrication precision required with increased wafer size and intricate device geometry has resulted in higher clean room standards and a need for particle detection devices to ensure protection from even smaller particles which might contaminate the production environment.
As a result of increased clean room standards and industry demands, more, lower cost laser detection devices have been introduced which can detect particles down to a fraction of a micron in size. A semiconductor laser or similar laser is commonly used to generate the laser beam in such detection devices. U.S. Pat. No. Re. 33,213 to Borden teaches the benefits of using a laser beam to generate a sheet of light in a particle detector.
Some prior art laser detection devices have placed air samples directly into the laser cavity. Such systems produce a.c. and d.c. noise due to molecular scattering. As the amount of noise increases, it becomes difficult to distinguish between the noise caused by the molecular scattering, and light scattered by mobile particles. As a result, such devices require noise reduction systems. Additionally, such prior art devices require larger lasers, and limit the user's ability to remove or change the laser.
Therefore, it is an object of this invention to provide a particle detection device with high sensitivity, but with a high signal-to-noise ratio, and a simple construction.